Method for production and forming of cast pieces of spheroidal graphite with improved mechanical properties

ABSTRACT

The invention relates to a method of preparing and forming parts of spheroidal graphite cast iron having high-grade mechanical characteristics. The method comprises the following steps: a) preparing a mixture in the liquid state comprising by weight: 3% to 4% C; 1.7% to 3% Si; 0.1% to 0.7% Mn; 0 to 4% Ni; 0 to 1.5% Cu; 0 to 0.5% Mo, 0.025% to 0.080% Mg, the balance being iron and impurities resulting from preparation; b) casting the mixture in the liquid state at a temperature of from 1350° C. to 1550° C. into a mold c) extracting a blank of the part from the mold at a temperature between solidus and AR3; d) forming the blank of the part by hot plastic deformation; the forming taking place at a temperature of from 1050° C. and AR3; e) quenching said part directly in the heat of forming at a temperature in the bainitic range, and maintaining said temperature; and f) cooling said part to ambient temperature.

The present invention relates to a method of preparing and formingcastings of spheroidal graphite cast iron with high-grade mechanicalcharacteristics, and to the cast iron as obtained by implementing themethod.

More precisely, the invention relates to a method of preparing andforming castings of spheroidal graphite cast iron having high toughness,i.e. high mechanical strength. The term “high toughness” is usedtypically to designate cast irons presenting traction strength in therange 1000 megapascals (MPa) to 1700 MPa, a ratio of elastic limit overtraction strength Rp0.2/Rm≧0.68, and breaking elongation lying in therange about 4% to about 14%.

Advances in certain techniques make it necessary for cast irons, or moregenerally materials, to be available that have high-gradecharacteristics, capable of withstanding higher and higher mechanicalstresses. This applies in particular in the automotive industry becauseof the increasing performance of motor vehicle engines, itself leadingto an increase in the levels of stress applied to various mechanicalmembers and components.

At present, numerous grades of molded cast iron are available for suchmechanical applications. They represent a very wide variety of ranges ofcharacteristics adapted to various kinds of stress and they oftenconstitute materials that are competitive over other materials, and inparticular over high performance forged steels, for manufacturingmembers that are subjected to very high stresses.

Nevertheless, in certain applications, economic constraints due tocompetition are more and more severe and it is therefore particularlyadvantageous to be able to extend the range of construction materialscapable of satisfying very severe conditions of use and meeting theexpectations of manufacturers for making various parts.

To achieve this object, the invention provides a method of preparing andforming parts of spheroidal graphite cast iron having high-grademechanical characteristics, the method being characterized in that itcomprises the following steps:

-   -   a) preparing a mixture in the liquid state having the following        composition by weight: 3% to 4% C: 1.7% to 3% Si; 0.1% to 0.7%        Mn; 0 to 4% Ni; 0 to 1.5% Cu; 0 to 0.5% Mo, with a residual Mg        content adapted to the thickness of the parts lying in the range        0.025% to 0.080%, the balance being iron and impurities        resulting from preparation; the impurities being in particular S        at a content of less than 0.015%, and P at a content of less        than 0.10%;    -   b) casting the mixture in the liquid state at a temperature        lying in the range 1350° C. to 1550° C. into a mold enabling a        shape approaching that of the workpiece, which is referred to as        the blank of the part;    -   c) extracting said blank of the part from the mold at a        temperature Ts lying between the solidus and AR3, the solidus        and AR3 representing the temperatures defining the austenitic        region of said composition;    -   d) forming the blank of the part by hot plastic deformation        directly in the heat of casting, in order to obtain the part        with its final shape and dimensions, the forming taking place at        a temperature Tf lying in the range 1050° C. and AR3;    -   e) quenching said part directly in the heat of forming at a        temperature Tb lying in the bainitic range, and maintaining it        at said temperature Tb for a duration tb; and    -   f) cooling said part to ambient temperature.

The term “quenching” or “bainitic staged quenching” is used here andthroughout the text below in the broad sense, i.e. it relates to coolingthat is fast enough to obtain a structure with practically noferrito-pearlite and with practically no martensite either.

Quenching or bainitic staged quenching is intended to confer anessentially bainitic structure to the workpiece, i.e. constituted bymore than 50% bainite.

For the grades of cast iron implemented using said composition, the rateof cooling corresponding to bainitic staged quenching lies in the range15° C./sec to 150° C./sec.

In this first implementation of the method of the invention, and also inthe following implementation when Ts is greater than Tf, all of thesuccessive steps needed for making the cast iron workpiece from the stepof casting to the step of cooling the workpiece to ambient temperatureare performed in the heat of casting, i.e. without intermediatereheating between said steps. These two implementations of the method ofthe invention without intermediate reheating between the successivesteps needed for making the cast iron workpiece provide particularlyhigh performance in terms of energy consumption compared withconventional heat treatment and forming methods. They are thereforeparticularly inexpensive for preparing spheroidal graphite cast ironwith high-grade mechanical characteristics.

In another implementation of the method of the invention, between stepc) and step d), i.e. between the blank being extracted from the mold andthe blank being formed by plastic deformation while hot, an additionalstep is performed in which the blank is maintained at a temperature Tmthat is 200C to 500C higher than the forging temperature Tf for aduration lying in the range 10 min to 60 min in order to ensure thattemperature within the blank is uniform, when the temperature Ts atwhich the blank is extracted from the mold is greater than thetemperature Tf that is desired for forging; or when the temperature Tsat which the blank is extracted from the mold is lower than thetemperature Tf that is desired for forging, in which case the blank isheated and maintained at the temperature Tm that is 20° C. to 50° C.higher than the forging temperature Tf for a duration lying in the range10 min to 60 min.

In another implementation of the method of the invention, step e) forimplementing direct bainitic staged quenching is replaced by one of thetwo following treatments:

-   -   1) after hot plastic deformation, the workpiece is cooled into        the intercritical region at a temperature Tir lying in the range        AR1+20° C. and AR3, it is maintained at this temperature Tir for        a duration lying in the range 15 min to 60 min to allow its        structure to homogenize, and bainitic staged quenching is        performed at a temperature Tb situated in the bainitic region,        and the workpiece is maintained at a temperature Tb for a        duration tb;    -   2) after hot plastic deformation, the workpiece is allowed to        cool to below the temperature AR1, and then the workpiece is        reheated and maintained at a temperature Tic lying in the range        AC1+20° C. to AC3, i.e. in the intercritical region for the        composition of the cast iron for a duration lying in the range        30 min to 180 min in order to allow the structure to homogenize,        and bainitic staged quenching is performed at a temperature Tb        situated in the bainitic region, and the workpiece is maintained        at the temperature Tb for a duration tb.

AR1, AR3, AC1, and AC3 represent the limits of the critical interval,also known as the intercritical region, of said composition as measuredrespectively during cooling (index R) and heating (index C); thetemperature Tic or Tir that is maintained in the intercritical region orthe critical interval lies in the range 740° C. to 850° C., depending onthe composition of the cast iron in question.

Treatment 2) is more expensive in energy terms than treatment 1), but isnevertheless preferable since it makes it easier to adjust the structureof the cast iron in terms of ferrite content and austenite content priorto bainitic staged quenching.

This implementation of the invention with treatment in the intercriticalregion prior to bainitic staged quenching serves to reduce the hardnessobtained on the workpieces, and thus makes them easier to machinesubsequently, where appropriate; it also encourages very high toughnessof the material by the presence of a larger proportion of ferrite in thestructure obtained for the cast iron after bainitic staged quenching. Infact, the structure is constituted under these circumstances by twovarieties of ferrite, a ferrite I resulting from the treatment in theintercritical region, and a ferrite II resulting from the treatment inthe bainitic region, together with austenite; the ferrite II and theaustenite represent the bainite.

In another implementation of the method of the invention, step e)consisting in performing bainitic staged quenching is preceded bytreatment of maintaining a temperature lying in the range 950° C. to900° C. for a duration lying in the range 15 min to 60 min for thepurpose of making the temperatures of the various portions of theworkpiece more homogeneous and thus making its chemical composition morehomogeneous prior to the following bainitic staged quenching treatment.

Finally, in another implementation of the method of the invention, anadditional step is added of cold calibration of the workpiece after stepf) of cooling the workpiece to ambient temperature, which calibration isperformed between at least two matrices having the shape of the finishedpart in order to improve its dimensional precision and increase themechanical characteristics and the fatigue strength of the material ofthe workpiece by work-hardening its surface; prior to this operation ofcold calibration, shot-blasting is performed, which serves to removescale and to generate surface compression stresses serving to furtherreinforce the effect of the work-hardening due to calibration.

The various implementations of the method of the invention for preparingand forming parts made of spheroidal graphite cast iron enable cast ironto be obtained of structure that can be essentially bainitic or that canpresent a structure constituted by two varieties of ferrite: ferrite I,and ferrite II with austenite, the ferrite I being that which resultsfrom remaining in the critical interval at the temperature Tir or Tic,and the ferrite II being that which results from treatment in thebainitic region at the temperature Tb, and having mechanicalcharacteristics at 20° C. that are typically traction strength Rm lyingin the range 1000 MPa to 1700 MPa, a ratio of elastic limit overtraction strength: Rp0.2/Rm greater than 0.68, and a breaking elongationcoefficient lying in the range 4% to 14%. The temperature Tic or Tirmaintained in the intercritical region or in the critical interval liesin the range 740° C. to 850° C., depending on the composition of thecast iron in question.

The term “hot plastic deformation” as used above and below designatesdeformation at a ratio lying overall in the range 2% to 60%.

The best results are obtained for a good strength-to-toughnesscompromise corresponding to hot plastic deformation ratios lying in therange 20% to 50% depending on the forging temperatures used in the range1050° C. to AR3 of the cast iron, since at above 50%, large deformationis observed of the graphite nodules that is harmful for the mechanicalcharacteristics. The deformation ratios given herein are defined asbeing the difference between the initial thickness of the section of theworkpiece subjected to forging by flattening and the final thicknessthereof after forging, relative to the initial thickness of said sectionof the workpiece and multiplied by 100 in order to express the result asa percentage.

The term “hot plastic deformation” or the term “forging” means above andbelow mainly an operation of stamping at a temperature greater than AC3or AR3 of said cast iron composition, but also designates other forms ofhot plastic deformation implemented at a temperature higher than AC3 orAR3 of said cast iron composition, such as free forging, stamping,rolling, hydroforming, etc.

In order to perform this hot plastic deformation in the context of theinvention, it is necessary for the blank of the molded workpiece, whichblank is approximately in the shape of the part, to possess at least oneof its dimensions that is greater than that of the part proper so as tomake said hot plastic deformation possible.

Preferably, the mold used for casting the workpiece blank is a permanentmold constituted by at least two metal half-portions coated in a releaseagent, however the mold may also be a semi-permanent sand moldconstituted by at least two sand shell mold portions placed in a metalmold or it could also be a non-permanent mold of chemical sand or ofgreen silica-clay sand.

When using sand molds, the workpiece blank must be hot-brushed orhot-sanded to remove grains of sand adhering thereto, prior toperforming the forging operation.

The bainitic staged quenching temperature Tb lies in the range 260° C.to 420° C. This temperature Tb is preferably selected to lie in therange 260° C. to 300° C. when it is desired to have a high Rp0.2/Rmratio together with high traction strength Rm; conversely, when it isdesired to have traction strength Rm close to 1000 MPa or 1100 MPa, Tbshould be selected to be greater than 300° C.

In order to obtain a properly-formed structure containing bainiteconstituted by ferrite and austenite saturated in carbon, the durationtb at which the bainitic staged quenching temperature Tb is maintainedshould preferably lie in the range 60 min to 180 min.

The invention also provides spheroidal graphite cast iron havinghigh-grade mechanical characteristics prepared and formed using themethod of the invention, and having a structure that is essentiallybainitic, with the following mechanical characteristics at 20° C.:

-   -   traction strength Rm: 1000 MPa≦Rm≦1700 MPa;    -   ratio of elastic limit Rp0.2 over traction strength Rm:        Rp0.2/Rm≧0.68; and    -   breaking elongation A: 4%≦A≦14%.

Finally, the invention provides a spheroidal graphite cast iron withhigh-grade mechanical characteristics prepared and formed byimplementing the method of the invention, of structure made up of twovarieties of ferrite: ferrite I and ferrite II together with austenite,the ferrite I being the result of spending time in the critical intervalat the temperature Tir or Tic, and the ferrite II being the result oftreatment in the bainitic region at the temperature Tb, the cast ironhaving the following mechanical characteristics at 20° C.:

-   -   traction strength Rm: 1000 MPa≦Rm≦1700 MPa;    -   ratio of elastic limit Rp0.2 over traction strength Rm:        Rp0.2/Rm≧0.68; and    -   breaking elongation A: 4%≦A≦14%.

The present invention is particularly adapted, but not restricted, tofabricating automotive engine parts such as connecting rods for exampleor other moving parts of the engine. Lighter in weight and havingmechanical characteristics that are equal to or better than those of theforged steel presently used in engines, connecting rods made ofspheroidal graphite cast iron with high-grade mechanical characteristicsprepared and formed by the method of the invention can enable the engineto have improved energy efficiency and thus achieve savings in its fuelconsumption.

Other characteristics and advantages of the present invention appearbetter on reading the following description of two implementations ofthe invention given by way of non-limiting example.

Tests have been performed on various testpieces of spheroidal graphitecast iron having the following compositions: Ref. C % Si % Mn % Ni % Cu% Mo % Mg % 1 3.54 2.61 0.17 traces 1.02 traces 0.040 2 3.6 2.40 0.120.7 0.9 0.15 0.042 3 3.72 2.42 0.24 traces 0.6 traces 0.045 4 3.7 2.700.13 0.3 0.5 0.28 0.045

In accordance with the invention, these cast irons referenced 1 to 4were prepared in an electric induction furnace, treated withferro-silico-magnesium, and they were cast at a temperature lying in therange 1460° C. to 1500° C. into a metal mold of temperature regulated at280° C. and coated in a protective release agent.

Five testpiece blanks were cast using each cast iron composition, thetestpieces being in the form of rectangular bars having the followingdimensions: Section in Reference of blank millimeters (mm) Length in mmA 20 × 10 150 B 20 × 12 150 C 20 × 15 150 D 20 × 18 150 E 20 × 24 150

For each mold casting, a full set of testpiece blanks was made. Fourcastings were undertaken per composition.

In an implementation of the invention, the testpiece blanks wereextracted from the mold at a temperature that was found throughouttesting to lie in the range 1000° C. to 980° C., and they were placedimmediately in a bath comprising a fluidized bed of silica sand at atemperature regulated at 980° C. so as to ensure the same forgingtemperature for each blank.

Ten minutes after being placed in the fluidized bed bath at a regulatedtemperature of 980° C., the blanks were withdrawn one after another atintervals of about 10 seconds (sec) and they were formed by hot plasticdeformation by being stamped between two matrices having the shape ofthe testpiece hollowed out therein. Each blank was thus plasticallydeformed by stamping at a temperature lying in the range 960° C. to 940°C. in order to bring the final section of each testpiece to 20 mm×10 mm.

Under such conditions, the deformation ratio to which each testpieceblank was subjected was as follows: Initial Final Blank section sectionDeformation ratio reference in mm in mm in % A 20 × 10 20 × 10 0% B 20 ×12 20 × 10 (12 − 10)/12 × 100 = 16.7% C 20 × 15 20 × 10 (15 − 10)/15 ×100 = 33.3% D 20 × 18 20 × 10 (18 − 10)/18 × 100 = 44.4% E 20 × 24 20 ×10 (24 − 10)/24 × 100 = 58.3%

Immediately after stamping, each testpiece was subjected to a deburringoperation in a press and was immediately placed in a fluidized bed bathof zircon sand at a regulated temperature of 300° C. and of sufficientlylarge volume to guarantee temperature variation in the fluidized bed ofless than 5° C. when the various testpieces were immersed therein atintervals of about 10 sec. Each testpiece was thus subjected to bainiticstaged quenching at the temperature of 300° C. and was maintained atsaid temperature in the fluidized bed for a duration of 110 min, afterwhich each testpiece was withdrawn from the fluidized bed bath andallowed to cool in air down to ambient temperature in accordance withthe invention.

The table of FIG. 1 shows the results obtained. This table shows thatthe expected mechanical characteristics were indeed obtained, i.e.:

-   -   traction strength Rm: 1000 MPa≦Rm≦1700 MPa;    -   ratio of elastic limit Rp0.2 over traction strength Rm:        Rp0.2/Rm≧0.68; and    -   breaking elongation A: 4%≦A≦14%; with an essentially bainitic        structure constituted of ferrite and austenite.

Another series of tests was undertaken in accordance with anotherimplementation of the invention using the cast iron compositionreferenced 2: in this other test, the conditions under which thetestpiece blanks were prepared and the testpieces themselves were thesame as in the above test except that an additional step of maintainingthe testpieces at a temperature in the intercritical region wasperformed between the stamping and deburring operation and the operationof bainitic staged quenching. After forging and deburring in the samemanner as in the above test, the testpieces were placed immediately in afluidized bed at a regulated temperature Tic of 810° C., situated in theintercritical region and lying in the range AC1+20° C. to AC3 for aduration of 60 min, after which the testpieces were quenched in thebainitic region in the fluidized bed at a regulated temperature Tb of300° C. and they were maintained at that temperature for a duration of110 min, as in the above test, and under exactly the same conditions. Atthe end of that bainitic staged quenching treatment, the testpieces werewithdrawn from the fluidized bed bath and allowed to cool in air down toambient temperature, in accordance with the invention.

The table of FIG. 2 shows the results obtained. This table shows thatthe expected mechanical characteristics were indeed obtained, i.e.:

-   -   traction strength Rm: 1000 MPa≦Rm≦1700 MPa;    -   ratio of elastic limit Rp0.2 over traction strength Rm:        Rp0.2/Rm≧0.68; and    -   breaking elongation A: 4%≦A≦14%;        with a structure made up of two varieties of ferrite: a ferrite        I resulting from being maintained in the intercritical region at        the temperature Tic or Tir, and a ferrite II with austenite, the        ferrite II coming from the treatment in the bainitic region        performed at the temperature Tb.

1-20. (canceled)
 21. A method of preparing and forming parts ofspheroidal graphite cast iron having high-grade mechanicalcharacteristics, the method comprising the following steps: a) preparinga mixture in the liquid state having the following composition byweight: 3% to 4% C; 1.7% to 3% Si; 0.1% to 0.7% Mn; 0 to 4% Ni; 0 to1.5% Cu; 0 to 0.5% Mo, with a residual Mg content adapted to thethickness of the parts lying in the range 0.025% to 0.080%, the balancebeing iron and impurities resulting from preparation; the impuritiesincluding S at a content of less than 0.015%, and P at a content of lessthan 0.10%; b) casting the mixture in the liquid state at a temperatureof from 1350° C. to 1550° C. into a mold enabling a shape approachingthat of the workpiece, which is referred to as the blank of the part; c)extracting said blank of the part from the mold at a temperature Tsranging from the solidus to AR3, wherein the solidus and AR3representing the temperatures defining the austenitic region of saidcomposition; d) forming a blank of the part by hot plastic deformationdirectly in the heat of casting, in order to obtain the part with itsfinal shape and dimensions, the forming taking place at a temperature Tfof from 1050° C. to AR3; e) quenching said part directly in the heat offorming at a temperature Tb lying in the bainitic range, and maintainingthe part at said temperature Tb for a duration tb; and f) cooling saidpart to ambient temperature.
 22. A method of preparing and forming partsof spheroidal graphite cast iron having high-grade mechanicalcharacteristics, the method comprising the following steps: a) preparinga mixture in the liquid state having the following composition byweight: 3% to 4% C; 1.7% to 3% Si; 0.1% to 0.7% Mn; 0 to 4% Ni; 0 to1.5% Cu; 0 to 0.5% Mo, with a residual Mg content adapted to thethickness of the parts lying in the range 0.025% to 0.080%, the balancebeing iron and impurities resulting from preparation; the impuritiesincluding S at a content of less than 0.015%, and P at a content of lessthan 0.10%; b) casting the mixture in the liquid state at a temperatureof from 1350° C. to 1550° C. into a mold enabling a shape approachingthat of the workpiece, which is referred to as the blank of the part; c)extracting said blank of the part from the mold at a temperature Tsranging from the solidus and AR3, the solidus and AR3 representing thetemperatures defining the austenitic region of said composition; c′)maintaining the blank at a temperature Tm that is 20° C. to 50° C.higher than a forging temperature, for a duration of from 10 min to 60min, so as to ensure that the temperature inside the blank ishomogeneous; d) forming a blank of the part by hot plastic deformation,in order to obtain the part with its final shape and dimensions, theforming taking place at a temperature Tfoffrom 1050° C. to AR3; e)quenching said part directly in the heat of forming at a temperature Tblying in the bainitic range, and maintaining the part at saidtemperature Tb for a duration tb; and f) cooling said part to ambienttemperature.
 23. A method according to claim 22, wherein when thetemperature Ts at which the blank is extracted from the mold is lowerthan the temperature Tf desired for forging, the blank is heated andmaintained at the temperature Tm that is 20° C. to 50° C. greater thanthe forging temperature Tf for a duration of from 10 min to 60 minduring step c′).
 24. A method of preparing and forming parts ofspheroidal graphite cast iron having high-grade mechanicalcharacteristics, the method comprising the following steps: a) preparinga mixture in the liquid state having the following composition byweight: 3% to 4% C; 1.7% to 3% Si; 0.1% to 0.7% Mn; 0 to 4% Ni; 0 to1.5% Cu; 0 to 0.5% Mo, with a residual Mg content adapted to thethickness of the parts lying in the range 0.025% to 0.080%, the balancebeing iron and impurities resulting from preparation; the impuritiesincluding S at a content of less than 0.015%, and P at a content of lessthan 0.10%; b) casting the mixture in the liquid state at a temperatureof from 1350° C. to 1550° C. into a mold enabling a shape approachingthat of the workpiece, which is referred to as the blank of the part; c)extracting said blank of the part from the mold at a temperature Tsranging from the solidus and AR3, the solidus and AR3 representing thetemperatures defining the austenitic region of said composition; d)forming a blank of the part by hot plastic deformation directly in theheat of casting, in order to obtain the part with its final shape anddimensions, the forming taking place at a temperature Tf of from 1050°C. to AR3; e) cooling the part in the intercritical region at atemperature T_(ir) of from AR1+20° C. to AR3, the part being maintainedat said temperature T_(ir) for a duration of from 15 min to 60 min toallow its structure to become homogeneous, and performing bainiticstaged quenching at a temperature Tb situated in the bainitic region,and maintaining the part at the temperature Tb for a duration tb, AR1,AR3, AC1, and AC3 representing the limits of the critical interval or ofthe intercritical region of said composition as measured respectively oncooling (index R) and on heating (index C); and f) cooling said part toambient temperature.
 25. A method according to claim 24, wherein afterthe part has been formed by hot plastic deformation in step d), saidpart being allowed to cool to below the temperature AR1, and thenheating the part and maintaining the part at a temperature T_(ic) offrom AC1+20° C. to AC3, the intercritical region of the composition ofthe cast iron, for a duration of from 30 min to 180 min in order toallow the part's structure to become homogeneous, and bainitic stagedquenching is performed at a temperature Tb situated in the bainiticregion and the part is maintained at the temperature Tb for a durationtb, AR1, AR3, AC1, and AC3 representing the limits of the criticalinterval of said composition as measured respectively on cooling (indexR) and on heating (index C); and said part is cooled to ambienttemperature.
 26. A method according to claim 21, wherein the stepcorresponding to bainitic staged quenching of the part at a temperatureTb is preceded by a treatment step comprising maintaining the part at atemperature of from 950° C. to 900° C. for a duration of from 15 min to60 min for the purpose of making the temperatures of the variousportions of the part homogeneous and making the part's chemicalcomposition homogeneous prior to the bainitic staged quenchingtreatment.
 27. A method according to claim 21, wherein an additionalstep of cold calibration of the part is added after the step of coolingthe part to ambient temperature, the additional step being performedbetween at least two matrices having the shape of the finished part inorder to improve its dimensional precision and to increase themechanical characteristics and the fatigue strength of the material ofthe part by work-hardening its surface.
 28. A method according to claim27, wherein prior to the operation of cold calibration, a shot-blastingoperation is performed for the purpose of removing scale and deliveringcompression stresses to the surface enabling the work-hardening effectdue to calibration to be reinforced.
 29. A method according to claim 21,wherein the mold used for casting the blank of the part is a permanentmold constituted by at least two metal half-portions coated in a releaseagent.
 30. A method according to claim 21, wherein the rate of coolingcorresponding to bainitic staged quenching lies in the range 15° C./secto 150° C./sec.
 31. A method according to claim 21, wherein thetemperature T_(ic) or T_(ir) at which the part is maintained in theintercritical region or critical interval lies in the range 740° C. to850° C.
 32. A method according to claim 21, wherein the hot plasticdeformation performed at a temperature greater than AC3 or AR3 for saidcast iron composition comprises a stamping operation, an operation offorging, rolling, or hydroforming.
 33. A method according to claim 21,wherein the hot plastic deformation is deformation at a ratio lyingoverall in the range 2% to 60%.
 34. A method according to claim 21,wherein the cast blank of the part is of a shape approaching that of thepart and possessing at least one dimension that is greater than that ofthe part so as to make said hot plastic deformation possible.
 35. Amethod according to claim 21, wherein the temperature Tb of the bainiticstaged quenching lies in the range 260° C. to 420° C.
 36. A methodaccording to claim 21, wherein the duration tb for which the temperatureTb of bainitic staged quenching is maintained is from 60 min to 180 min.37. Spheroidal graphite cast iron having high-grade mechanicalcharacteristics, and having the following composition by weight: 3% to4% C; 1.7% to 3% Si; 0.1% to 0.7% Mn; 0 to 4% Ni; 0 to 1.5% Cu; 0 to0.5% Mo, with a residual Mg content adapted to the thickness of theparts lying in the range 0.025% to 0.080%, the balance being iron andimpurities resulting from preparation; the impurities including S at acontent of less than 0.015%, and P at a content of less than 0.10%,characterized in that its structure is essentially bainitic. 38.Spheroidal graphite cast iron having high-grade mechanicalcharacteristics, and having the following composition by weight: 3% to4% C; 1.7% to 3% Si; 0.1% to 0.7% Mn; 0 to 4% Ni; 0 to 1.5% Cu; 0 to0.5% Mo, with a residual Mg content adapted to the thickness of theparts lying in the range 0.025% to 0.080%, the balance being iron andimpurities resulting from preparation; the impurities including S at acontent of less than 0.015%, and P at a content of less than 0.10%,characterized in that its structure is made up of two varieties offerrite and austenite.
 39. A cast iron part prepared from the spheroidalgraphite cast iron according to claim
 37. 40. A method according toclaim 21, wherein all of the successive steps needed for making the castiron part from the casting step to the step of cooling the part toambient temperature are performed without intermediate heating betweensaid steps.
 41. A method according to claim 22, wherein an additionalstep of cold calibration of the part is added after the step of coolingthe part to ambient temperature, the additional step being performedbetween at least two matrices having the shape of the finished part inorder to improve its dimensional precision and to increase themechanical characteristics and the fatigue strength of the material ofthe part by work-hardening its surface.
 42. A method according to claim41, wherein prior to the operation of cold calibration, a shot-blastingoperation is performed for the purpose of removing scale and deliveringcompression stresses to the surface enabling the work-hardening effectdue to calibration to be reinforced.
 43. A method according to claim 22,wherein the mold used for casting the blank of the part is a permanentmold constituted by at least two metal half-portions coated in a releaseagent.
 44. A method according to claim 22, wherein the rate of coolingcorresponding to bainitic staged quenching lies in the range 15° C./secto 150° C./sec.
 45. A method according to claim 22, wherein thetemperature Tic or T_(ir) at which the part is maintained in theintercritical region or critical interval lies in the range 740° C. to850° C.
 46. A method according to claim 22, wherein the hot plasticdeformation performed at a temperature greater than AC3 or AR3 for saidcast iron composition comprises a stamping operation, an operation offorging, rolling, or hydroforming.
 47. A method according to claim 22,wherein the hot plastic deformation is deformation at a ratio lyingoverall in the range 2% to 60%.
 48. A method according to claim 22,wherein the cast blank of the part is of a shape approaching that of thepart and possessing at least one dimension that is greater than that ofthe part so as to make said hot plastic deformation possible.
 49. Amethod according to claim 22, wherein the temperature Tb of the bainiticstaged quenching lies in the range 260° C. to 420° C.
 50. A methodaccording to claim 22, wherein the duration tb for which the temperatureTb of bainitic staged quenching is maintained is from 60 min to 180 min.51. A method according to claim 24, wherein an additional step of coldcalibration of the part is added after the step of cooling the part toambient temperature, the additional step being performed between atleast two matrices having the shape of the finished part in order toimprove its dimensional precision and to increase the mechanicalcharacteristics and the fatigue strength of the material of the part bywork-hardening its surface.
 52. A method according to claim 51, whereinprior to the operation of cold calibration, a shot-blasting operation isperformed for the purpose of removing scale and delivering compressionstresses to the surface enabling the work-hardening effect due tocalibration to be reinforced.
 53. A method according to claim 24,wherein the mold used for casting the blank of the part is a permanentmold constituted by at least two metal half-portions coated in a releaseagent.
 54. A method according to claim 24, wherein the rate of coolingcorresponding to bainitic staged quenching lies in the range 15° C./secto 150° C./sec.
 55. A method according to claim 24, wherein thetemperature T_(ic) or T_(ir) at which the part is maintained in theintercritical region or critical interval lies in the range 740° C. to850° C.
 56. A method according to claim 24, wherein the hot plasticdeformation performed at a temperature greater than AC3 or AR3 for saidcast iron composition comprises a stamping operation, an operation offorging, rolling, or hydroforming.
 57. A method according to claim 24,wherein the hot plastic deformation is deformation at a ratio lyingoverall in the range 2% to 60%.
 58. A method according to claim 24,wherein the cast blank of the part is of a shape approaching that of thepart and possessing at least one dimension that is greater than that ofthe part so as to make said hot plastic deformation possible.
 59. Amethod according to claim 24, wherein the temperature Tb of the bainiticstaged quenching lies in the range 260° C. to 420° C.
 60. A methodaccording to claim 24, wherein the duration tb for which the temperatureTb of bainitic staged quenching is maintained is from 60 min to 180 min.61. A method according to claim 22, wherein the step corresponding tobainitic staged quenching of the part at a temperature Tb is preceded bya treatment step comprising maintaining the part at a temperature offrom 950° C. to 900° C. for a duration of from 15 min to 60 min for thepurpose of making the temperatures of the various portions of the parthomogeneous and making the part's chemical composition homogeneous priorto the bainitic staged quenching treatment.
 62. A method according toclaim 23, wherein the step corresponding to bainitic staged quenching ofthe part at a temperature Tb is preceded by a treatment step comprisingmaintaining the part at a temperature of from 950° C. to 900° C. for aduration of from 15 min to 60 min for the purpose of making thetemperatures of the various portions of the part homogeneous and makingthe part's chemical composition homogeneous prior to the bainitic stagedquenching treatment.
 63. A method according to claim 24, wherein thestep corresponding to bainitic staged quenching of the part at atemperature Tb is preceded by a treatment step comprising maintainingthe part at a temperature of from 950° C. to 900° C. for a duration offrom 15 min to 60 min for the purpose of making the temperatures of thevarious portions of the part homogeneous and making the part's chemicalcomposition homogeneous prior to the bainitic staged quenchingtreatment.
 64. A method according to claim 25, wherein the stepcorresponding to bainitic staged quenching of the part at a temperatureTb is preceded by a treatment step comprising maintaining the part at atemperature of from 950° C. to 900° C. for a duration of from 15 min to60 min for the purpose of making the temperatures of the variousportions of the part homogeneous and making the part's chemicalcomposition homogeneous prior to the bainitic staged quenchingtreatment.
 65. A cast iron part prepared from the spheroidal graphitecast iron according to claim
 38. 66. A method according to claim 22,wherein all of the successive steps needed for making the cast iron partfrom the casting step to the step of cooling the part to ambienttemperature are performed without intermediate heating between saidsteps.